chore(storage): storage file structure changes

[qdm12]

Description

• migrate persisted server data storage from /gluetun/servers.json to /gluetun/servers/
• add STORAGE_SERVERS_ENABLED=on to enable or disable on-disk server data storage
• add STORAGE_SERVERS_DIRECTORY_PATH=/gluetun/servers to configure where per-provider server files are stored
• keep backward compatibility with legacy STORAGE_FILEPATH=/gluetun/servers.json
• automatically read and migrate legacy /gluetun/servers.json into the new /gluetun/servers/ layout when needed
• try to remove the legacy servers file after a successful migration to the new storage directory
• switch persisted server data from one large JSON file to a manifest plus per-provider JSON files
• add UPDATER_PREFER_DIRECT_DOWNLOAD to allow preferring direct download of provider server data
• keep deprecated updater flags -enduser and -maintainer as no-op warnings for backward compatibility
• preserve compatibility checks so persisted server data is discarded when its schema version no longer matches the built-in data
• allow preferred persisted provider data to override built-in data when versions match
• servers data now lives at https://github.com/qdm12/gluetun-servers/tree/main/pkg/servers

Later:

• set UPDATER_PREFER_DIRECT_DOWNLOAD=yes

[qdm12]

We need to integrate the workflow from #3307 in gluetun-servers